In semiconductor wafer substrate (wafer) cleaning, particle removal is essential. Particles can be removed by chemical means or by mechanical means. In the current state of art, one means of removing particles includes the use of a megasonic cleaning device. A megasonic cleaning device utilizes a process wherein a wafer is placed in a liquid bath and high frequency (megasonic) irradiation, or cavitation, is applied to the liquid in the bath. At the same time, chemicals in the liquid provide a slight surface etching and provide the right surface termination, such that once particles are dislodged from the surface by the combination of etch and mechanical action of the megasonics on the particles, these particles are not redeposited on the surface.
Various types of megasonic cleaning devices vary in diverse ways. Some types can clean one wafer at a time. Other types utilize very clever ways to reduce the amount of liquid used in the bath. Yet other types work at some frequencies better than others. Nonetheless, almost all megasonic cleaning devices are similar in at least one way. They all make use of an acoustic-wave transducer.
An acoustic-wave transducer is an electronic device that receives high-frequency power signals (from an RF power source) that excite the transducer and cause it to vibrate. The vibration causes sonic waves to travel through the liquid bath and provide the mechanical means to remove particles from the wafer surface.
For several reasons, however, power signals are not always successfully delivered to the acoustic-wave transducer in the most efficient manner. To properly deliver power to an acoustic-wave transducer, the resistance of the power source must match that of the transducer load. However, for the transducer to function properly, the power signals must be of a sinusoidal nature, such as alternating current (AC). As a consequence of utilizing AC, reactive circuit elements, such as the transmission lines within the megasonic cleaning device, create impedance, and impedance matching is more difficult to accomplish than mere resistance matching.
Some approaches have nonetheless been attempted to match the impedance of an acoustic-wave transducer load to the impedance of an RF power source. One common approach has been to utilize an impedance-matching transformer. A transformer is an electronic device with two wires wound around a magnetic core. The wires wound around the core are called “windings.” Typically, the coil connected to the AC power source is called the primary winding, while the coil connected to the load is called the secondary windings. The number of windings in the primary coil compared to the number of windings in the secondary coil is called the “turns ratio.” A well known quality of the transformer is that the turns ratio squared is proportional to the impedances of circuits connected to the primary and secondary windings as follows: (NP/NS)2; ZP/ZS, where NP is the number of primary windings, NS is the number of secondary windings, ZP is the impedance seen at the input of the transformer on the primary side and ZN is the impedance seen at the input of the transformer on the secondary side.
Thus, one approach for matching impedances of transducer loads to that of an RF power source has been to connect the RF power source to the primary windings of a transformer, connect the transducer load to the secondary windings, then to manipulate the turns ratio until the impedances are matched as seen from both sides. This approach, however, has serious detrimental side effects.
For example, the transformer, itself, introduces a good deal of inductance into the circuit, thus increasing the reactance of the circuit. As the reactance goes up, so does the impedance. Impedance is measured in two parts, however, magnitude and phase. Unfortunately, while the transformer works to correct impedance magnitude, it also changes phase. Impedance phase can also affect power loss between the power source and the transducer load. In time, as electronic devices on a silicon wafer decrease in size, so do the particles that need cleaning. As a result, the transducer assembly must create cavitation at higher frequencies. A common way to create higher cavitation frequencies is to use a larger transducer. The larger transducer, however, the lower the impedance it provides, and, as a result, the higher the turns ratio must be in the transformer to match the power source impedance. As the turns ratio increases in the transformer, so does the inductance, and, consequently, so does the phase-difference in the power signals. The more the phase signals get out of phase, the less efficient is the power transfer.